240 volt electric heaters?

I bought an electric heater with cord and plug (the cord is 6 feet, 14 gauge) and thermostat (Marley NPH4A) rated at 240 volts, 4000 watts (high), 16.7 amps. The outside of the box shows a 6-20P plug, but the instructions (internet manual) shows a 6-30P plug (the actual manual enclosed was for a 5000 watt heater ?) The unit will be used on a single circuit at a receptacle 42 feet from the electrical panel. I'm hung up on the continuous load question. Could a 25 amp breaker be used on 10-2 wire and would a 20 amp-250v receptacle be used?